Grid controlled mercury arc tube



' Oct. 29, 1935. J E AN 2,019,277

GRID CONTROLLED MERCURY ARC TUBE Filed May 27, 1955 Va/VepaSs/ny V 9 curren/ a7 Vol/aye:

' 4601 2 a crz'h'cal value WITNESSES: I v INVENTOR Joseph Shep/an.

0%. ATTO R N Patented Oct. 29, 1935 UNITED STATES PATENT OFFICE GRID CONTROLLED MERCURY ARC TUBE sylvania Application May 27, 1933, Serial No. 673,237

9 Claims.

My invention relates to gaseous electrical discharge devices and in particular to gaseous electrical discharge devices arranged to-have current flow therethrough controlled by means of a grid certainty that has never before been attained.

Another object of my invention is to provide for controlling current flow through a gaseous electrical discharge device-by means of control electrodes, in which the magnitude of the controlled voltage and/or current which can be governed with certainty by a given control voltage is greater than in similar devices of the prior art.

A further object of my invention is to provide an arrangement by means of which a current and/or voltage between the principal electrodes of a gaseous electrical discharge device can be controlled with certainty with less expensive control circuit arrangements and simpler and less expensive control currents than those characterizing the prior art.

A further object of my invention is to provide an arrangement of control electrodes adapted to govern the current flowing between the principal electrodes of a mercury arc discharge device, which shall perform its expected functions with greater regularity and certainty than control electrode arrangements of the prior art.

Other objects of my invention will become apparent upon reading the following specification, in which The single figure of drawing illustrates, diagrammatically, a mercury arc discharge tube embodying my invention.

In accordance with prior art, it has been the practice to control current flow between one or more anodes and cathodes in gaseous electrical discharge devices by controlling the potential of a grid or other auxiliary electrode interposed between the principal electrodes. Thus a control grid, interposed between the anode and the excited cathode of a mercury arc tube, has been arranged to have its potential relative to the anode made. negative whenever it was desired to prevent current flow between anode and cathode, and to have its potential made more positive whenever it was desired that such current flow should be. Similarly, a grid electrode interposed between an anode and a thermionically emissive cathode of a gaseous discharge tube has had its potential controlled relative to the cathode, to permit current more positive. Sources of periodically pulsating voltage have usually been used to supply current to the main anode in such arrangements.

However, experiment has shown that a grid electrode sometimes fails to function to prevent 5 current fiow even when circuits are arranged which supposedly impressed a negative potential on the control electrode relative to the cathode. This failureto operate successfully as a control appears to be somewhat erratic, and there are inl dications that it is due to a momentary disappearance or substantial lessening of the degree of negative potential of the control electrode relative to the cathode. Cumulative ionization phenomena in the gaseous atmosphere and sudden tran- 15 sient changes in surface conditions along insulators within the discharge tube, akin to the mechanism of voltage breakdown in gases, appear as probable causes of these efiects.

However, these random phenomena are probably of very short duration in time, and the probability that they will efiect two separate control electrodes simultaneously is extremely remote. In accordance with the principles of my invention,

therefore, I provide two or more grids or other 16 control electrodes interposed between the principal electrodes of a gaseous discharge tube, either of which control electrodes would be of a character to govern by its current passage between the two main electrodes. Only in the very improbl0 able occurrence of all such control electrodes being subject simultaneously to the random loss of governing power, would this electrode aggregate lose control over the main current flow. Likewise,

I provide in series with each separate control It electrode, in accordance with the principles of my invention, an inductance shunted by a circuit element which has the property of freely passing electrical discharges when subjected to an electrical potential above a certain critical value and of imposing a much higher impedance to electrical flow when the potential across its terminals falls below a critical value.

Referring in detail to the figure of drawing, a vacuum-tight electrical discharge tube I, which 15 may be of glass, is provided with a. principal anode 2, a cathode 3, and a side branch 4, all of which may be types well-known in the mercury arc rectifier art. Between the anode 2 and the cathode 3 are arranged two control electrodes 5, 6 50 which may conveniently be meshes or grids of nickel or other wire, and provided respectively, with in-leading wires I, 8. For many fields of use it will be preferable that one, or in some cases both, the electrodes 5, s be within a distance of ll the anode which is less than the mean-free-path of an electron in the atmosphere of the tube. Also a separation between electrodes 5, 6 of less than a mean-free path of an electron is within the provision of my invention. The control electrodes may for some uses comprise a hood about the anode with a plurality of electrically separate partitions containing a single hole, or a plurality of holes of such diameter that a positiveion sheath about the edge of the hole substantially blocks the entire area to electron flow when the control electrodes is at low potential relative to the cathode. Between the anode 2 and the. oath-. ode 3 is connected a work-device a supplied from a source of electromotive force I I, which is pref erably of an alternating or other periodically pulsating type. One terminal of a source of variable control and biasing voltage 12 is con? nected to the cathode 3 and between the other terminal of the source l2, and each of the control electrodes 5, B is connected inductances l3, l4 shunted by valves IE, it.

The principles which determine how large the inductances I3 and I4 need to be are fully discussed in my copending patent application, Serial No. 548,719 filed July 4, 1931 and assigned to the Westinghouse Electric and Manufacturing Company. Briefly they should be so large that during the time of existence of the causes of low voltage on the grid, the current which can flow into the grid will be kept so low that a permanent arc will not be established. We may take .010 amperes as too low for a permanent arc, and hence we may say that the inductance must be large enough to keep the current below .010 amperes for the duration of a cause of low voltage on the grid. If we take this duration as 10- seconds, and if for example the voltage applied to the grid is volts, then the inductance required will be or 0.1 henries. Since a properly laminated iron core may be used, there is no difficulty in obtainingreactances of this magnitude. The distributed capacity of these inductances l3, I4 should preferably be kept to a low value or minimized. Cou-. plings, of the inductances or the circuits ofcontrol electrodes 5, 6 so that the decrease of negative voltage on one electrode will cause a reaction tending to increase the negative voltage on the other is an advantageous feature which it maybe desirable to employ in some cases.

The valves l5, l6 may be of any type suitedto discharge accumulated electric charges without permitting the subsequent flow of power currents, a function which lightning arresters are usually adapted to perform. Specifically, I prefer to use lightning arresters of the type described either in Patent No. 1,873,362 or Patent No. 2,000,719. The omission of such an impedance net from the lead of one control electrode would be contemplated by my invention; as would also the omission of one or both the valves I5, 16.

As an example of the effectiveness of my invention, suppose that random effects cause each grid electrode to lose its controlling efiect for a transient period of 10 seconds, and to occur once in every ten minutes, then such efiects will occur on both grids simultaneously only once in minutes, that is to say, once in 114 years of continuous operation.

While I have described my invention as applied to a mercury arc tube having a cathode excited by a side branch, it will be evident to those skilled in the art that for many purposes the invention is applicable to a tube having mercury electrodes which are not excited; and, in fact, to gaseous discharge tubes with cold electrodes generally. It will also be evident that the invention is par- 1 ticularly applicable to gaseous electrical discharge tubes having one or both electrodes maintained at a high temperature to. cause them to emit electrons f reely. Similarly where so-called highvacuum discharge devices having therimomcally 1 emissive photo-emissive or other electro-emissive electrodes, i. e. devices in which the gas-pressure is so low that normally current is not conducted by ions substantially, are subjected to high potentials, multiple control-electrodes may be em- 1 ployed in accordance with my invention.

While {have described my invention in connectiorr with the particular embodiment here illustrated, it will be evident to those skilled in the art that many departures from the proposed 5 structure and process of construction which I have described may be made without departing from the principles of my invention. 1, therefore, desire that the terms of the claims shall be accorded their broadest reasonable significa- I tion excepting so far as limitations are expressly madeor are imposed by the prior art.

I claim as my invention: a

1. An electrical discharge device comprising a plurality of main electrodes, a plurality of con-v I trol electrodes for each pair of main electrodes, a current limiting impedance in serial relation with each control electrode, and a single source of biasing potential for each pair of control elec: trodes.

2 An electrical discharge device. comprising a plurality of main electrodes, a plurality of control electrodes for each pair of main electrodes. and an impedance connected to said control electrodes and one of said main electrodes including I means adapted to oppose the free flow of current at voltages impressed, across its terminals below a critical value and to permit free flow of current at voltages substantially above said critical value.

3. An electrical discharge device comprising a plurality of main electrodes including an excitable cathode, a plurality of control electrodes foreach pair of main electrodes, a current limiting. impedance in serial relation with each. control electrode and a single source of biasing po.-. tential for each pair of control electrodes.

4. electrical discharge device comprising a plurality of main electrodes, a plurality or control electrodes for each pair of main electrodes, an impedance in serial relation with each control electrodeand means adapted to draw ofi an electric charge without permitting a substantial flow of power current shunting said impedances.

5. An electrical discharge device comprising a plurality of main electrodes, a plurality of. control electrodes for each pair of main electrodes, an impedance in serial relation with eachcontrol electrode means adapted to draw ofi an electriccharge without permitting a substantial flow of power shunting said impedances and a single source of. biasing potential for said control electrodes.

6. A gaseous electrical discharge device having a pluralitv of main electrodes, a plurality, or

control electrodes for each pair of main electrodes and a current limiting impedance in serial relation with each control electrode.

'1. A gaseous electrical discharge device having a plurality of main electrodes including an excitable cathode, a plurality of control electrodes for each pair of main electrodes, an inductance in serial relation with each control electrode and one oi said main electrodes and means adapted to oppose the free flow 01' current at voltages impressed across its terminals below a critical value and to permit free flow of current at voltages substantially above said critical value shunting said inductances.

8. A gaseous electrical discharge device having a plurality of main electrodes including an excitable cathode, an inductance in serial relation with each control electrode and means adapted to draw ofi an electric charge without permitting a substantial flow of power shunting said inductance.

9. An electrical discharge device comprising a container, a gaseous medium therein, a plurality of main electrodes and a control electrode therein, an inductance connected between said control electrode and one of said main electrodes and means shunting said inductance adapted to oppose the free flow of current at voltages impressed across its terminals below a critical value and to permit free flow of current at voltages substantially above said critical value.

JOSEPH SLEPIAN. 

